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Putting Hydrogen on Ice

Researchers looking for better ways to make and store hydrogen have accidentally discovered an entirely new kind of ice. Made of molecular oxygen and hydrogen, the highly energetic and as-yet-unnamed compound currently exists only under rarefied laboratory conditions. It is different from the 17 known forms of ice, but researchers think its discovery could advance understanding of the nature of water under extreme conditions, such as in the interior of planets and even inside nuclear reactors. It also might help to spawn new rocket fuels.

A team at the Carnegie Institution of Washington led by Wendy Mao had been attempting to split apart water at high pressures to form a solid mixture of molecular oxygen and molecular hydrogen. The researchers squeezed a sample of water with a diamond anvil to a pressure of 17 gigapascals, about 170,000 times the normal atmospheric pressure at sea level. They then bombarded the water with high-energy x-rays. The molecules split, then reformed into a previously unknown combination of O2 and H2. The key to splitting the oxygen-hydrogen bonds was the strength of the x-rays. “We managed to hit on just the right level,” says team member Russell Hemley. Using a higher energy, he says, would have caused the x-rays to pass entirely through the sample. Any lower, however, and the diamonds in the pressure apparatus would have largely absorbed the radiation.

Hemley says the narrow energy range explains why no one had discovered the compound before. Previous experiments used shorter bursts of more energetic x-rays, whereas this attempt used a lower energy level and a much longer period of bombardment. As long as the team sustained the x-rays at 10,000 electron volts and maintained a pressure of at least 1 gigapascal, the new substance retained its integrity. Reporting in today's Science, the researchers say the compound is a crystalline solid, but they need further data to determine its precise structure. “The observations do tell us something about the mechanism of breakdown of water to make hydrogen and oxygen,” Hemley says. “Such information is ultimately important for understanding and perfecting processes for making hydrogen and oxygen from water over a broad range of conditions.”

“This is brand new stuff and very exciting,” says physical chemist Richard Saykally of the University of California, Berkeley. Although researchers had thought for some time that water could form exotic compounds under extreme conditions, this is the first demonstration of such a material, he says. Saykally adds that the new compound might interest NASA and the U.S. Air Force, which are looking for more efficient oxydizers for solid fuel rockets, and this one “must have incredibly high energy” to exist.